In wheel suspensions of the above-described type, often known as McPherson struts, the upper transverse control arm present in otherwise conventional wheel suspension is replaced by a long-stroke spring leg (see Lueger, "Lexikon der Technik" [Lexicon of Technology], Vol. 12, "Lexikon der Fahrzeugtechnik" [Lexicon of Automotive Engineering], 1967, Deutsche VerlagsAnstalt GmbH, Stuttgart, page 425).
Helical compression springs are always embodied in such a way that the spring center line is a straight line; any deviations from this are unintentional and dictated by manufacturing processes, but are tolerated within standard limits (see DIN 2096, part 2, paragraph 8). The line of spring force action coincides with the spring center line.
In wheel suspensions of the type that is the point of departure for the invention, the helical compression spring can be disposed initially concentric with the shock absorber axis. Then the entire shear force at the upper support point, that is, occurring at the body, must be absorbed by the piston rod of the shock absorber. This generates considerable frictional forces at the piston of the shock absorber, so that spring compression and relaxation is jerky.
To reduce the shear forced acting upon the shock absorber piston rod, the helical compression spring has for a long time been built into the spring leg in such a way that the line of spring force action forms an acute angle with the shock absorber axis. In the ideal case, the angle between the line of spring force action of the helical compression spring and the shock absorber axis would have to correspond to the angle between the line of supporting action and the shock absorber axis. Then in the normally loaded state, the piston rod of the shock absorber would be largely free of shear forces. Only upon compression or deflection--away from the static position of equilibrium, equal to the normally loaded state--would shear forces arise. In general, however, the angle between the line of spring force action of the helical compression spring and the shock absorber axis can be made only smaller than the angle between the line of support action and the shock absorber axis, because the smooth passage of the shock absorber through the helical compression spring and sufficient freedom of motion of the wheel must still be assured.
Because tires are becoming wider and wider, hence shifting the wheel-to-road contact point outward, larger and larger angles between the line of support action and the shock absorber axis arise, to which the line of spring force action must be adjusted, if the piston rod of the shock absorber is to be free of shear forces in the normally loaded state.
Since for the above reasons the helical compression spring cannot be positioned as obliquely with respect to the shock absorber axis as would actually be desired, the line of spring force action in the prior art has already been shifted with respect to the spring center line, by placing one or both end windings obliquely, by thickening the end windings, by placing the spring plate obliquely, or by a combination of these provisions. This makes it possible, within limits, to locate the spring force action line farther outward than the spring center line at the lower end of the helical compression spring, while at the upper end of the helical compression spring the line of spring force action and the spring center line pass through the support points. However, it is still unsatisfactory that the attainable angle between the line of spring force action and the shock absorber axis is still inadequate, and that the helical compression spring, in the medium operating range, in compression and relaxation, tends to have its windings contacting one another and tends to bulge outward.